1. Field of the Invention
This invention relates to adhesive compositions, particularly co-extrudable adhesives, suitable for complex composite structures, such as those having multiple barrier and structural layers which are typically incompatible with each other. The adhesive compositions are blends of acid-grafted polyethylene and unmodified polyethylene and styrene butadiene rubber.
2. Discussion of Related Art
Co-extrudable adhesives based on blends of various polyethylenes which also contain an acid-grafted polyolefin to aid in adhesion to polar layers are well known.
U.S. Pat. No. 3,868,433 discloses polyolefins generally, graft-modified with acids, and which may also contain elastomers, for use as hot-melt adhesives.
U.S. Pat. No. 4,684,576 discloses adhesive blends based on acid-grafted high density polyethylene, and linear low density polyethylene of density 0.88 to 0.935.
U.S. Pat. No. 4,058,647 discloses blends of modified and unmodified polyolefins with a rubber component having 40-150 of Mooney viscosity 50 ML 1+4 at 100.degree. C. There is no teaching of elastomeric components having immeasurable Mooney viscosities under these conditions for blending with the polyolefins nor is there a specific example which shows or describes the use of a styrene based elastomer.
U. S. Pat. 4,198,327 discloses compositions of graft modified crystalline polyolefins (optionally mixed with ungrafted crystalline polyolefins) and a hydrocarbon elastomer as having improved adhesion to polar solid materials. While styrene butadiene rubber is generically disclosed-there is no specific teaching in the examples of the use of a styrene based elastomer.
EPO 412 503 A2 discloses a multitude of adhesive resin compositions and multilayer structures containing the adhesives. An adhesive composition of a second embodiment as disclosed therein relates to a composition having 100 parts by weight of a soft polymer and 1 to 30 parts by weight of partially or wholly grafted modified polyethylene in which a grafting amount of an unsaturated carboxylic acid or a derivative thereof is 0.05 to 15% by weight, the soft polymer comprising (a) 20 to 100% by weight of a styrene elastomer and (b) 80 to 0% by weight of an ethylene/alpha-olefin copolymer having an ethylene content of 45 to 95 mole %. Example 5 in this publication describes the use of 100 parts by weight ("pbw") of a stryene based elastomer and 10 pbw of maleic anhydride grafted polyethylene wherein this adhesive is layered between polycarbonate/EVOH/PP. Example 6 of this publication describes the use of 75 pbw of a styrene elastomer; 25 pbw of an ethylene/propylene random copolymer and 10 pbw of a maleic anhydride grafted PE and a multilayer structure similar to above or comprising PET/EVOH/PP with the adhesive between each internal layer-i.e., where the / is. This patent clearly specifically shows that a large percentage of styrene elastomer is necessary in the adhesive composition in spite of generic teachings which may suggest otherwise. In addition, the density of the unmodified ethylene-alpha olefin utilized therein in the blend with the styrene elastomer to form the soft polymer composition must be 0.850 to 0.900 g/cm. See also, U.S. Pat. Nos. 5,225,482 and 5,250,349.
U.S. Pat. No. 5,591,792 discloses adhesive compositions containing styrene-butadiene elastomers wherein the composition requires a tackifier as a key excipient.
U.S. Pat. 5,643,999 discloses mixtures of polyethylenes and certain elastomers wherein the blend may be modified or unmodified and wherein the adhesives are used to bond ionomer structural layers to barrier layers. This patent requires the use of at least two different polyethylenes with two distinct (non-overlapping) density ranges. No specific examples describe the use of a styrene based elastomer even though they (among other hydrocarbon elastomers with certain heats of fusion) are generically disclosed as comprising 5-30% of the composition. The present invention, on the other hand, requires that the styrene based elastomer is unmodified. In addition, the '999 patent teaches that no polyethylenes of density greater than 0.935 can be present in the composition.
U.S. Pat. No. 4,298,712 discloses adhesive blends having elastomers, grafted high density polyethylene and polyethylene resin(s). The grafting reagent selected to graft to the high density polyethylene is selected from unsaturated fused ring carboxylic acid anhydrides. This compounds must contain one or more carbocyclic or other ring moieties not including the anhydride ring.
It is also known that polyolefins, such as polyethylene and polypropylene, are widely used as packaging materials because of their good mechanical strength, water resistance and organoleptic properties. Because of their high permeability to gases, such as oxygen and carbon dioxide, etc., such polyolefins are frequently combined with polar polymers with excellent barrier properties, such as ethylene vinyl alcohol polymers and polyamides, to form a composite structure with desirable properties.
Such composite structures are held together by acid modified extrudable adhesives. It is a relatively simple matter to select such adhesives when the composite structure contains very similar structural layers. A structure comprising of polypropylene and EVOH or polypropylene and nylon can easily be bound together with a acid modified polypropylene adhesive. Similarly, a structure of polyethylene and EVOH can be bonded together with an acid-modified polyethylene adhesive. It becomes a more complicated matter when the composite structure contains different structural layers. For example, with a composite film of polyethylene/nylon/polypropylene, one could use an acid-modified polyethylene adhesive to bond the polyethylene layer to the nylon layer and an acid-modified polypropylene adhesive to bond the polypropylene layer to the nylon layer. This could be highly inconvenient to the converter since it would necessitate the use of two separate adhesive extruders rather than a single adhesive extruder. Alternatively, the fabricator could use an acid modified ethylene vinyl acetate or ethylene methyl acrylate adhesive to bond both polypropylene and polyethylene layers to the nylon layers. However, such adhesives are well known to impart odor and taste to packaged foods; thus, the excellent organoleptic properties of the composite structure are compromised.
In another example, a composite structure of polypropylene, EVOH and ethylene ionomer cannot be bound well together by a single acid modified polyethylene or polypropylene adhesive and the converter, again, must resort to either using both adhesives or to an acid modified ethylene vinyl acetate or ethylene methyl acrylate adhesive.
It is an object of this invention, therefore, to provide a polyolefin based adhesive composition that can be used to bond polypropylene as part of at least a three-layer multilayer structure to ethylene polymers, such as polyethylene, ethylene acid copolymers and ethylene acid ionomers without requiring two different adhesives while also able to bond similar substrates through an intervening barrier layer such as polypropylene/nylon/polypropylene. In addition, the compositions of the invention do not require a separate tackifier as an essential ingredient and in some embodiments, as compositions and in certain multilayer structures having said compositions, ethylene-alpha olefins having a density of less than or equal to 0.900 g/cm3 are excluded. The inventors have found that medium to high density unmodified polymers coupled with a combination of modified polymers and styrene based elastomers in certain relative weight percentages provides significant gains in adhesion properties. Surprisingly, the inventors have also found that the particular choice of elastomer in the adhesive blend has a significant effect on adhesive properties.